There is a hidden factor in the evolution of human beings which is neither a 'missing link' nor a telos imparted from on high. I suggest that this hidden factor in the evolution of human beings, the factor which called human consciousness forth from a bipedal ape with binocular vision, involved a feedback loop with plant hallucinogens. This is not an idea that has been widely explored, though a very conservative form of this notion appears in R. Gordon Wasson's 'Soma: Divine Mushroom of Immortality' (Wasson, 1971). Wasson does not comment on the emergence of human-ness out of primates, but does suggest hallucinogenic mushrooms as the causal agent in the appearance of spiritually aware human beings and the genesis of religion.
The state of consciousness would provide a reason for foraging humans to return repeatedly to those plants, in order to re-experience their bewitching novelty. The primate gains increased visual acuity and access to the transcendent Other, ever more novel information and sensory input and behaviour, and thus is bootstrapped to higher and higher states of self-reflection.